For peace of mind in anyones home, these loud battery operated alarms are a must. Simple to attach to any window or door using the self adhesive backing provided and has a simple On/Off switch to the side. Once an attempted forced entry is made on a window or door the alarm will detect unusual vibrations made by the intruder and will activate, emitting a high pitched 95dB sound. Each purchase buys you TWO alarms. Each alarm requires 3 x LR44 button cell batteries (not included). They may come with batteries but they may be past their shelf life so you must use new batteries upon receipt. Dimensions are approx. 6cm in diameter and 2cm in thickness.